Some carbon emissions are very straightforward to calculate. The chemical process of burning a litre of pure petrol releases known quantities of carbon dioxide, methane, nitrous oxide and other gases. Broadly speaking, these just have to be converted to CO2e values and added up. Later, this number can be multiplied by the volume of petrol used to get a fairly accurate figure for the emissions produced by a tank of fuel.

However these straightforward figures are of limited use in the real world. 

The fuel you put in your car is hardly ever pure petrol - UK garages sell a product that is blended with biofuels. These blends vary widely - from different garage chains or refineries, for the "standard" or "performance" pump, or because of the changing prices of raw materials. Each tank of "petrol" produces a slightly different mix of greenhouse gases and so has a slightly different CO2e per litre.

Capturing the exact makeup of every tank of "petrol" would mean asking users to enter a lot of data for each receipt. Instead, we need a figure for the average carbon emissions for each litre of real-world fuel. This needs to take account of the chemistry of the different blends on sale in the UK, and the proportion of each type of fuel that people actually buy.

This is where the DEFRA factors come in. The UK government funds scientists and statisticians to collate this data, turn it into robust statistical models, and produce a simple figure of CO2e for each litre of "Petrol (average biofuel blend)". This value changes each year, based on changes in the UK petrol market. A similar process is done for Diesel.

This means we just need to ask users to enter a number of litres and whether it's "petrol" or "diesel" - claimexpenses can then calculate a rough carbon emission figure. There will of course be a slight discrepancy, depending on whether you bought a fuel that is better or worse than the UK average.

Litres of fuel is one of the easiest factors. Others are harder to establish. You may only know how far you drove, not how much petrol you burned. So the government produces factors to calculate carbon emissions based on vehicle mileage, considering things such as:

• the average composition of the fuels people buy, as above
• the tested fuel economy of all the various makes, models and ages of car on the road.
• the proportion of all the miles driven that are in each particular type of car - this uses data from numberplate recognition and traffic surveys
• the proportion of all the miles driven on different types of roads (in town / motorway / etc)
• "real-world" fuel consumption exceeding the tested values because of things like using air conditioning, driving style, weather, hills, poor maintenance, presence of passengers and luggage etc.

These statistics are broken down into various broad classes of vehicles, for example kg CO2e per mile when travelling in a "small car". This of course has a much higher potential discrepancy. A cautious driver travelling alone in a modern small car will produce lower emissions than the DEFRA figures, while a more aggressive driver with a fully laden older car will produce more.

Emissions factors for other transport modes are more complex again. Train journeys, for example, need to take account of the split between diesel and electric trains, the breakdown of the fuels used to make the electricity, and the numbers of passengers that share the emissions of each train. Flights likewise are affected by passenger numbers, fuel mix, congestion, and the altitude they fly at (carbon released higher in the atmosphere has a more damaging effect than carbon nearer the ground).

The DEFRA factors allow claimexpenses.com to produce robust approximations of carbon emissions, without having to ask you to enter excessive amounts of detail.

As explained above, it would be virtually impossible to calculate precise carbon emissions for each journey to the last gram. The calculations would be complex, and would require you to enter huge volumes of data. It would also be subject to a lot of variation over time and between systems according to the methodology or small differences in some of the assumptions that would be required.

It's also not particularly important that the carbon emissions figure precisely matches the actual emissions. The main considerations are that it:

• Roughly correlates to the relative damage of different modes of transport - the value for a flight should be a lot higher than the value for a train going the same distance
• Is broadly consistent with the calculations other organisations use - a company that flies 2 people from Edinburgh to London for a meeting should produce roughly twice the carbon of an organisation that sends one in person and one by video conference.
• Is broadly consistent with calculations in the past - a company that halves the number and distance of flights compared to the previous year should have roughly halved their carbon emissions
• Reflects broad changes in the carbon impact of different types of travel over time - if electric trains start to use more renewable power the carbon emissions of rail travel should fall, if airlines run more half-empty flights the carbon emissions of flying should increase, as older cars are scrapped emissions from driving should fall.
• Is based on a set of assumptions and measurements that are reasonably rigorous and repeatable

The DEFRA factors are carefully designed to achieve these objectives. They produce figures that are precise enough to drive, and reward, behavioural change and emissions reduction. For this reason, they're widely accepted as the standard method of calculating emissions from business processes in the UK.

Of course, an organisation might take initiatives that reduce real-world emissions but don't show up in figures calculated from the DEFRA averages. You might send people on a fuel efficient driving course, or require staff to prioritise rail routes with electric trains. These are obviously positive steps. However, in many cases they will not make a big enough difference to the organisation's overall annual emissions to justify the work involved in calculating the precise reduction.

If your organisation rolled out carbon reduction initiatives on a large scale, one option would be to produce a model comparing how your policy compared to the assumptions in the DEFRA factors. If, say, you only refunded receipts for petrol with a high biofuel content you could fairly easily calculate the difference between this and the UK average fuel mix. You could then capture the individual fuel purchases using the DEFRA averages, then apply your correction factor when you come to report on your emissions. 

Using correction factors like this requires caution - you would need to consider whether other areas of the business might be performing worse than the UK average. If a lot of journeys are on mostly-empty diesel trains, it would probably not be appropriate to reduce the CO2e for the petrol in your cars without also increasing the CO2e for your rail travel.

• Short ferries tend to go directly port-to-port, but longer routes (e.g. Scotland to Spain) may detour quite a bit from the straight line if there are other countries in the way.
• There will be lots of possible road routes between two places. Google's directions take account of things like road closures and congestion at the time of data entry to pick a route. Drivers might have a personal preference for a route that is longer than the one Google thinks is "best".
• Although UK rail lines are often fairly close to the best road route, they sometimes take quite a different path - going round mountains, but through city centres. Equally people may pick an indirect or longer route to get a better ticket price, or to fit their schedule.
• In other countries, there can be much bigger differences between rail and road journeys. Purpose-built autoroutes and high-speed rail lines often travel through completely different parts of the country, through different cities, etc.
• Large underground and tram networks often roughly follow the streets above, but for example going round the Clockwork Orange in Glasgow will be quite a bit longer than driving straight across the city.

If you have a more accurate distance - e.g. from a mileage reading in your car - you can override the estimate we produce to get a more accurate calculation. We always recommend doing this if you're claiming reimbursement for mileage as the estimated distances are almost certainly not accurate enough for finance purposes.

Equally, in most cases the error in the estimates will tend to average out over time unless you have large numbers of people regularly making the same trip.

It's also worth bearing in mind that - as explained above - the carbon emissions figures are always only going to be an approximation. Even if you go to the effort of calculating the exact mileage for every journey, the use of the DEFRA averages means you will not get an "exact" carbon figure.

The UK government - understandably - only produces emissions factors for journeys that are primarily in the UK. These factors may be less accurate for journeys in other countries - for example:

• The fuel mix sold and used by garages will be different
• There will be a different proportion of makes, models and ages of cars on the road
• Road speed limits and driving styles may be different
• There may be more, or fewer, electric trains - which may be older or newer than the ones operating in the UK
• The proportion of the country's electricity from low-carbon sources will be different
• Airspace may be less congested, so flights may travel more directly.
• There may be more short-haul commuter flights, with a different pattern of passenger numbers per aircraft

Carbon emissions for overseas journeys should therefore ideally be calculated with country-specific factors. Unfortunately, this is not currently possible because:

• Most countries don't have any sort of official publication that is as comprehensive or robust as the DEFRA factors
• Countries that do publish some emissions factors tend to break them down differently, so it would not be straightforward for claimexpenses to select the appropriate factor
• Journeys outside the UK are much more likely to cross country borders - for example a train from Calais to Warsaw might pass through France, Belgium and Germany before arriving in Poland. To use country-specific factors, we would need to know much more about the exact rail route to identify the distance travelled in each country.

These are issues that would first need to be resolved at the international level by governments or NGOS with the expertise and resources to develop a robust and comprehensive methodology.

However, for the purposes of the carbon emissions calculations on claimexpenses this is all largely academic - we simply use the DEFRA factors for all journeys. Our figures for non-UK journeys will be accurate to a lower level of precision than for journeys at home. But they will still, in general, meet the criteria set out in the "averaging and precision" section above. 

Going by train to Warsaw might in reality produce a bit more, or a bit less, CO2e than going the same distance by rail in the UK. But it will produce a lot less carbon than flying to Warsaw. The small percentage difference in the accuracy of the figures for the train journey will usually be less important than the much larger difference between the two modes of transport.

When actually planning trips, tools such as ecopassenger can be useful for comparing and optimising the carbon emissions of different routes and different modes of transport.

Of course, if you do significant travel in other countries, or micro-manage your travel to reduce carbon, you may want to explore more complex and comprehensive methodologies for measuring this. But most UK-based organisations should find the DEFRA factors provide a suitably robust, consistent and low-cost way emissions reporting framework.